In recent years, various techniques for producing a thermal sensor by using semiconductor microfabrication have been developed. A hollow structure in which a membrane provided with a detection electronic element is supported in a hollow state with respect to a substrate is in some cases adapted to a thermo sensor for the purpose of ensuring heat insulating property with respect to the substrate. An element having the structure where the membrane is supported in a hollow state is called a membrane structure element.
In general, a membrane is formed of a silicon oxide film such as a SiO2 (silicon dioxide) film that is excellent in heat insulating property, and such a film is easily formed by oxidizing a surface of a silicon substrate. The SiO2 film formed by the surface oxidation is called “heat oxidized SiO2 film”. However, since the heat oxidized SiO2 film has a heat expansion coefficient that is smaller than almost all of substrate materials such as a single crystal silicon, in the case where the hollow structure is formed by forming a heat oxidized SiO2 film to be used as a membrane, followed by cooling and then removing the substrate material below the heat oxidized SiO2 film by etching or the like to form a recessed part, the membrane supported in the hollow state may be brought into “sagged” state due to a large compression stress (about 200 MPa when the substrate is a single crystal silicon) remaining in the heat oxidized SiO2 film. Therefore, quality of the membrane structure element is deteriorated and strength of the membrane is deteriorated to entail a membrane breakage in the case of grave deterioration. Therefore, formation itself of a membrane having a large area becomes difficult.
Accordingly, various techniques for producing a membrane having a large area while minimizing an internal stress remaining in the membrane after membrane formation have been proposed. For example, JP-A-6-132277 (Patent Reference 1) discloses a technique for alleviating a remaining stress of a membrane as a whole by forming the membrane by laminating the SiO2 film and Si3N4 film having different thermal expansion coefficients to reduce an internal compressed remaining stress of a SiO2 film with the use of a Si3N4 film. Further, JP-A-8-264844 (Patent Reference 2) discloses a technique for reducing Young's modulus of a membrane by forming a central part of the membrane with the use of a Si3N4 film and adding a group V element to a SiO2 film. Also, Lie-yi Sheng et al., Transducers '97, 1997, pp. 939-942 (Non-Patent Reference 1) proposes a method of eliminating sagging of a membrane by using a polysilicon wiring serving also as a heater for a SiO2 film as “bones in a kite”.    Patent Reference 1: JP-A-6-132277    Patent Reference 2: JP-A-8-264844    Non-Patent Reference 1: Lie-yi Sheng et al., Transducers '97, 1997, pp. 939-942